Extensive Hepatic Uptake of Pz-peptide, a Hydrophilic Proline-Containing Pentapeptide, into Isolated Hepatocytes Compared with Colonocytes and Caco-2 Cells

The objective of the present study was to investigate the uptake process of 4-Phenylazobenzoxycarbonyl-Pro-Leu-Gly-Pro-D-Arg (Pz-peptide), a hydrophilic and collagenase-labile pentapeptide, by isolated hepatocytes. For comparison, the uptake of Pz-peptide by Caco-2 cells and colonic cells, two known paracellular routes of Pz-peptide, was also evaluated. A simple and sensitive reversed-phase HPLC assay method using UV detection has been developed. The coefficient of variation for all the criteria of validation were less than 15%. The method was, therefore, considered to be sutable for measuring the concentration of Pz-peptide in the biological cells. Pz-peptide was extensively uptaked into hepatocytes. The initial velocity of Pz-peptide uptake assessed from the initial slope of the curve was plotted as Eadie-Hofstee plots. The maximum velocity ($V_{max}$) and the Michaelis constant ($K_m$) were 0.190$\pm$0.020 $nmol/min/10^6$ cells and 12.1$\pm$3.23 $\mu$M, respectively. The permeability-surface area product ($PS{influx}$) was calculated to be 0.0157 ml/min/10^6$ cells. $V_{max}$ and $K_m$ values for Caco-2 cells were calculated to be 6.22$\pm$0.930 pmol/min/10^6$ cells and 82.8$\pm$8.37 $\mu$M, respectively, being comparable with those of colonocytes (6.04$\pm$1.03 pmol/min/10^6$ cells and 87.8$\pm$13.2 $\mu$M, respectively). $PS_{influx}$ values for Caco-2 cells and colonocytes were calculated to be 0.0751 $\mu$l/min/10^6$ cells and 0.0688 $\mu$l/min/10^6$ cells, respectively. The more pronounced uptake of Pz-peptide by hepatocytes, when compared with Caco-2 cells and colonocytes, is probably due to its specific transporter. In conclusion, Pz-peptide, a paracellularly transported pentapeptide in the intestine and ocular epithelia, was uptaked into hepatocytes extensively. Although Pz-peptide is able to be uptaked into the Caco-2 cells and colonocytes, it is less pronounced when compared with hepatocytes. $PS_{influx}$ values of Caco-2 cells and colonocytes for unbound Pz-peptide under linear conditions were less than 0.4% when compared with that of hepatocytes.